Inverter control method and system for eco-friendly vehicle

ABSTRACT

Provided is an inverter control system and method for an eco-friendly vehicle, by which overall improvements can be obtained in terms of switching loss, electromagnetic performance, noise-vibration-harshness (NVH) performance, control stability, and so forth, when compared to a conventional case in which one fixed switching frequency and one fixed sampling frequency are used over the entire operation area. To this end, the inverter control method for an eco-friendly vehicle which generates a pulse width modulation (PWM) signal according to a switching frequency and a sampling frequency and controls ON/OFF driving of a switching element, in which a controller changes and sets the switching frequency according to a current motor speed, changes and sets a sampling frequency according to the switching frequency, and controls on/off driving of a switching element according to the switching frequency corresponding to the motor speed and the sampling frequency.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Application No. 10-2012-0115174 filed on Oct. 17, 2012 and KoreanPatent Application Publication No. 10-2013-0050340 filed May 3, 2013,the entire contents of which are incorporated herein by reference and isa continuation in part of U.S. patent application Ser. No. 13/717,238,filed on Dec. 17, 2012.

BACKGROUND

(a) Technical Field

The present invention relates to an inverter control method and systemfor an eco-friendly vehicle, and more particularly, to an invertercontrol method and system for an eco-friendly vehicle, by which overallimprovements can be made in terms of switching loss, electromagneticperformance, noise-vibration-harshness (NVH) performance, controlstability, etc., when compared to a conventional control methods inwhich one fixed switching frequency and one fixed sampling frequency areused over the entire operation area.

(b) Background Art

As is well known, eco-friendly vehicles such as pure electric vehicles(EVs), hybrid electric vehicles (HEVs), fuel-cell electric vehicles(FCEVs), etc., use an electric motor as at least one driving source forvehicle driving.

In particular, direct-current (DC) power stored in a main battery of avehicle is transformed into three-phase alternating-current (AC) powervia an inverter between the battery and drive a motor, and a drivingforce of the motor is transferred to a driving wheel to allow vehicledriving.

In an eco-friendly vehicle, kinetic energy is transformed into electricenergy via regenerative braking during deceleration and the electricenergy is stored in a battery, and thereafter, while the vehicledriving, the energy stored in the battery is recycled back into drivingthe motor (e.g., the collected electric energy is recycled as kineticenergy to be utilized by the vehicle to, for example, recharge thebattery), thereby improving fuel efficiency.

The motor system, which typically includes a motor, which is operated asa driving source for an eco-friendly vehicle, and an inverter, hasseveral problems associated therewith such as a noise occurring duringdriving operation/regenerating operation, efficiency degradation causedby switching loss, electromagnetic performance degradation, and soforth.

Generally, if a switching frequency of an inverter increases, noisedecreases; as the switching frequency decreases, inverter efficiency andfuel efficiency may be improved.

That is, if the inverter's switching frequency is set to a low fixedfrequency (e.g., a base switching frequency is fixed to 4 kHz),electromagnetic performance may be good. However, a significant amountof noise is generated.

When the base switching frequency is set to be high over the entireoperational area to reduce the inverter's noise (for example, the baseswitching frequency is fixed to 8 kHz), NVH performance becomes better(i.e., pulse width modulation (PWM) current ripple is reduced), butelectromagnetic performance is deteriorated and switching loss increases(i.e., leading to degradation of heel hold performance in vehicleconstraint conditions), such that inverter efficiency and fuelefficiency are degraded as well.

As to electromagnetic performance, as the switching frequency increases,radiated electromagnetic noise increases (e.g., as a result, forexample, AM radio reception becomes poor); as the switching frequencydecreases, radiated noise decreases and thus electromagnetic performancebecomes better.

In a conventional eco-friendly vehicle, to reduce inverter noise whichmay be sensitively perceived or may displeasing to a driver or apassenger, the inverter's switching frequency is often set high andfixed (e.g., to 8 kHz) and sampling frequency for obtaining informationsuch as sensing current and (estimated) rotor position for controllingthe inverter is set equal to the switching frequency (8 kHz) (similarlywith the following single sampling scheme).

Herein, a switching frequency (i.e., switching period) may be defined asa period in which ON/OFF of a separate switch in the inverter isrepeated once, respectively, and a sampling frequency corresponds to acontrol period in controlling inverter's current, in which the controlperiod may be defined as a period of repeating rotor positioninformation, a current control operation, duty calculation, and a dutyupdate.

However, in a conventional case, one switching frequency is fixed andused over the entire operation area without any consideration of a motordriving conditions or the like (that is, a fixed frequency scheme isused), resulting in high switching loss caused by heat emission of aswitching element and in weakness of electromagnetic performance.

Moreover, when a sampling frequency is high, although inverter controlstability becomes better, a load on a processor executing the controlincreases because the processor has to obtain control parameters such assensing current, motor angular information, etc., in a shorter period oftime and calculate a larger number of control values the processor maybecome overloaded.

Therefore, it is necessary to control a switching frequency and asampling frequency according to a driving condition by considering NVHperformance, electromagnetic performance, switching loss, controlstability, a processor load factor, etc.

That is, in a conventional system, as the switching frequency is set andfixed at a high rate over the entire operational area, there areapparent disadvantages such as electromagnetic performance degradationand switching loss increase as well as some advantages. Consequently,there is a need for a control technique for properly changing aswitching frequency according to a driving condition for overallperformance improvement and properly adjusting a sampling frequencyaccording to the changed switching frequency.

SUMMARY OF THE DISCLOSURE

Accordingly, the present invention has been made to solve the foregoingproblem, and provides an inverter control system and method for aneco-friendly vehicle, by which overall improvement may be obtained interms of switching loss, electromagnetic performance, NVH performance,control stability, etc., when compared to a conventional control methodin which one fixed switching frequency and one fixed sampling frequencyare used over the entire operation area.

According to an aspect of the present invention, there is provided aninverter control system and method for an eco-friendly vehicle whichgenerates a pulse width modulation (PWM) signal according to a switchingfrequency and a sampling frequency and controls ON/OFF driving of aswitching element, in which a controller changes and sets the switchingfrequency according to a current motor speed, changes and sets asampling frequency according to the switching frequency, and controlson/off driving of a switching element according to the switchingfrequency corresponding to the motor speed and the sampling frequency.

According to another aspect of the present invention, there is providedan inverter control method for an eco-friendly vehicle which generates apulse width modulation (PWM) signal according to a switching frequencyand a sampling frequency and controls ON/OFF driving of a switchingelement, in which after a controller determines a base switchingfrequency according to a current motor speed, the controller changes andsets the switching frequency and a sampling frequency to valuescorresponding to a current motor operation state from the base switchingfrequency, and controls on/off driving of a switching element accordingto the switching frequency and the sampling frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to exemplary embodiments thereofillustrated the accompanying drawings which are given hereinbelow by wayof illustration only, and thus are not limitative of the presentinvention, and wherein:

FIG. 1 is a flowchart of an inverter control method according to a firstexemplary embodiment of the present invention;

FIG. 2 is a diagram showing a state where a switching frequency isvariable-controlled (continuous-variable-controlled) in an invertercontrol method according to a first exemplary embodiment of the presentinvention;

FIG. 3 is a diagram showing a transition scheme of a sampling frequencyin an inverter control method according to a first exemplary embodimentof the present invention;

FIG. 4 is a flowchart showing an inverter control method according to asecond exemplary embodiment of the present invention;

FIG. 5 is a diagram showing a state where a switching frequency isvariable-controlled (step-transition-controlled) in an inverter controlmethod according to a second exemplary embodiment of the presentinvention;

FIG. 6 is a diagram schematically showing that single sampling/doublesampling transition is made as step transition of a switching frequencyis made in an inverter control method according to a second exemplaryembodiment of the present invention;

FIGS. 7A and 7B are diagrams illustrating a base switching frequency anda spread frequency according to a third exemplary embodiment of thepresent invention;

FIG. 8 is a flowchart illustrating an inverter control method accordingto the third embodiment of the present invention;

FIG. 9 is a diagram illustrating a switching scheme for a samplingfrequency in an inverter control method according to the third exemplaryembodiment of the present invention;

FIG. 10 is a flowchart illustrating an inverter control method accordingto a fourth exemplary embodiment of the present invention;

FIGS. 11A and 11B are diagrams schematically illustrating a state inwhich a switching frequency is variable-controlled and singlesampling/double sampling transition is performed by step transition ofthe switching frequency in an inverter control method according to thefourth exemplary embodiment of the present invention;

FIG. 12 is a flowchart illustrating an inverter control method accordingto a fifth exemplary embodiment of the present invention;

FIGS. 13A and 13B are diagrams schematically illustrating a state inwhich a switching frequency is variable-controlled and singlesampling/double sampling transition is performed by step transition ofthe switching frequency in an inverter control method according to thefifth exemplary embodiment of the present invention; and

FIG. 14 is a diagram illustrating a step switch area of a switchingfrequency according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings to allow those of ordinary skillin the art to easily carry out the present invention.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuels derived fromresources other than petroleum). As to referred to herein, a hybridvehicle is a vehicle that has two or more sources of power, for exampleboth gasoline-powered and electric-powered vehicles.

Additionally, it is understood that the below methods are executed by atleast one controller. The term controller refers to a hardware devicethat includes a memory and a processor. The memory is configured tostore the modules and the processor is specifically configured toexecute said modules to perform one or more processes which aredescribed further below.

Furthermore, the control logic of the present invention may be embodiedas non-transitory computer readable media on a computer readable mediumcontaining executable program instructions executed by a processor,controller or the like. Examples of the computer readable mediumsinclude, but are not limited to, ROM, RAM, compact disc (CD)-ROMs,magnetic tapes, floppy disks, flash drives, smart cards and optical datastorage devices. The computer readable recording medium can also bedistributed in network coupled computer systems so that the computerreadable media is stored and executed in a distributed fashion, e.g., bya telematics server or a Controller Area Network (CAN).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

Unless specifically stated or obvious from context, as used herein, theterm “about” is understood as within a range of normal tolerance in theart, for example within 2 standard deviations of the mean. “About” canbe understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear fromthe context, all numerical values provided herein are modified by theterm “about.”

FIG. 1 is a flowchart of an inverter control method according to a firstexemplary embodiment of the present invention, and FIG. 2 is a diagramshowing a state where a switching frequency F_(sw) isvariable-controlled (continuous-variable-controlled) in an invertercontrol method according to a first embodiment of the present invention.

In FIG. 2, a conventional fixed switching frequency (e.g., about 8 kHz)is also shown.

FIG. 3 is a diagram showing a transition scheme of a sampling frequencyK_(samp) in an inverter control method according to the first exemplaryembodiment of the present invention, in which the sampling frequencyF_(samp) changes with the switching frequency F_(sw) and to transition(single sampling<-> double sampling) is performed in a particularcondition.

In first exemplary embodiment of the present invention, an inverterswitching frequency may be variable-controlled according to a vehicle'sdriving condition, and a sampling frequency may be properly controlledaccording to the changed switching frequency, thereby achievingswitching loss reduction and electromagnetic performance improvement.

First, a controller may be configured to monitor a current motor speedW_(rpm) and variably control the inverter switching frequency F_(sw)according to the current motor speed W_(rpm).

Herein, the motor speed W_(rpm) may be a motor speed calculated by aspeed calculator (differentiator executed by a processor within thecontroller) based on an absolute angular position θ detected by amotor's resolver.

In standard motor and inverter control, once the absolute angularposition θ—is detected by a resolver mounted on a motor, it is input tothe speed calculator which then calculates the motor speed W_(rpm) foruse in the control. Thus, in the exemplary embodiment, the controllermay variably control the switching frequency F_(sw) according to themotor speed W_(rpm).

The switching frequency F_(sw) is calculated as a value which changeswith the motor speed W_(rpm), such that as the motor speed W_(rpm)changes, the switching frequency F_(sw) also changes correspondingly.

Referring to FIG. 1, in step S11 in which the switching frequency F_(sw)is calculated according to the motor speed W_(rpm), the switchingfrequency F_(sw) is determined by a function of an absolute value of themotor speed W_(rpm), that is, |W_(rpm)|.

Once the changed value of the switching frequency is determinedaccording to the motor speed in this way, by using the changed switchingfrequency, through a known process of generating a pulse widthmodulation (PWM) signal, on/off driving of a switching element in aninverter (e.g., an insulated gate bipolar transistor (IGBT) of an IGBTpower module) which converts direct-current (DC) power into three-phasealternating-current (AC) power is controlled.

FIG. 2 is a diagram showing a continuous variable state of a switchingfrequency in which data information which previously defines theswitching frequency F_(sw) corresponding to the motor speed W_(rpm) (orfunctional formula (equation) information which defines a relationshipbetween a motor speed and a switching frequency) as shown in FIG. 2 maybe used to continuously change the switching frequency according to themotor speed, and by using this data information (previously storedwithin the memory in the controller), the controller calculates andchanges the switching frequency corresponding to the current motorspeed.

Referring to FIG. 2, a continuous increase or decrease pattern of theswitching frequency F_(sw) according to increase or decrease of themotor speed W_(rpm) is shown, and in the present invention, theinverter's switching frequency F_(sw) is determined by the motor speedW_(rpm) and the switching frequency F_(sw) is controlled to have apattern which continuously changes according to the motor speed W_(rpm).

When the switching frequency F_(sw) is determined and variablycontrolled according to the motor speed W_(rpm), as shown in FIG. 2, theswitching frequency is controlled to increase or decrease in proportionto an increase or decrease of the motor speed. That is, the switchingspeed corresponding to the motor speed is determined and controlled tobe a higher value as the motor speed increases (also as the motor speeddecreases, the switching frequency also decreases).

The data information of FIG. 2 is previously set such that as the motorspeed W_(rpm) (more specifically, an absolute value of the motor speed)increases, the switching frequency F_(sw) has a higher value, and incontrol of an actual vehicle, the switching frequency is continuouslychanged proportionally according to the motor speed change by using thepreviously set data information.

In FIG. 2, 8 kHz is a conventionally fixed base switching frequencyvalue. In the present invention, the switching frequency F_(sw) isvariably controlled below the conventional to base switching frequencyvalue according to driving conditions, i.e., the motor speed W_(rpm),such that in the motor's low speed area, the switching frequency isproperly reduced, thereby reducing switching loss and securingelectromagnetic performance.

The sampling frequency F_(samp) may be variably determined according tothe switching frequency F_(sw) determined by the motor speed W_(rpm),and the switching frequency F_(sw) may be compared with a previously setreference frequency F_(SD) in step S12. When the switching frequency islower than the reference frequency, the inverter is controlled in adouble-sampling mode (F_(samp)=2×F_(sw)) which uses a frequency that istwo times the switching frequency as the sampling frequency in step S13.

That is, if the switching frequency F_(sw) is lower than the referencefrequency F_(SD), the sampling frequency F_(samp) is determined to betwo times the switching frequency F_(sw), i.e., 2×F_(sw).

On the other hand, in an area where the switching frequency F_(sw)exceeds the reference frequency F_(SD), the sampling frequency F_(samp)is determined to be the same frequency as the switching frequency instep S14, and the inverter is controlled in a single-sampling mode(F_(samp)=F_(sw)) which uses the same frequency as the switchingfrequency as the sampling frequency.

Herein, the reference frequency serves as a criterion for transitionbetween the single-sampling mode (F_(samp)=F_(sw)) and thedouble-sampling mode (F_(samp)=2×F_(sw)), and the reference frequency ispreviously determined after a pre-test with respect to a motor systemhaving the same specifications.

Referring to FIG. 3, in an area where the switching frequency F_(sw) islower than the reference frequency F_(SD), and in an area where theswitching frequency F_(sw) exceeds the reference frequency F_(SD), theinverter is controlled in the double-sampling mode (step S13) and thesingle-sampling mode (step S14), respectively.

Since the switching frequency changes with increase or decrease in themotor speed, the sampling frequency also changes with increase ordecrease of the motor speed. However, double sampling and singlesampling are determined according to whether the switching frequency islower or higher than the reference frequency, and when the switchingfrequency increases or decreases around the reference frequency,discontinuous change of the sampling frequency, that is, mode transitionbetween double sampling and single sampling, occurs.

As such, in the first exemplary embodiment of the present invention, theswitching frequency continuously varies with the motor speed and modetransition between double sampling and single sampling is made based ona particular speed of the motor.

That is, the inverter is controlled in the double-sampling mode in whichthe sampling frequency is set two times the switching frequency, in thelow-speed area of the motor (in which the switching frequencycorresponding to the motor speed is less than the reference frequency),and in the single-sampling mode in which the sampling frequency is setto be equal to the switching frequency, in the high-speed area (i.e.,when the switching frequency exceeds the reference frequency).

In the present invention, the single-sampling mode may be defined as adigital control mode in which one control period occurs during oneswitching period, and the double-sampling mode may be defined as adigital control mode in which two control periods occur during oneswitching period. In the double-sampling mode, independent duty changefor each of an ON sequence and an OFF sequence may be possible.

With the control method according to the present invention, a lowerswitching frequency than with a conventional case can be used in thelow-speed area of the motor, thereby reducing switching loss andsecuring electromagnetic performance.

Next, a second exemplary embodiment of the present invention will bedescribed.

FIG. 4 is a flowchart showing an inverter control method according tothe second exemplary embodiment of the present invention, and FIG. 5 isa diagram showing a state where the switching frequency isvariable-controlled (step-transition-controlled) in the inverter controlmethod according to the second exemplary embodiment of the presentinvention.

FIG. 6 is a diagram schematically showing that single sampling/doublesampling transition is made as step transition of the switchingfrequency is made in the inverter control method according to the secondexemplary embodiment of the present invention.

As shown in FIG. 4, the controller monitors the current motor speedW_(rpm), and determines a base switching frequency F_(sw) _(—) _(base)according to the current motor speed W_(rpm) in step S11′.

The base switching frequency F_(sw) _(—) _(base) is determined as afunction of an absolute value of the motor speed W_(rpm), that is,|W_(rpm)|, and to this end, data information which previously defines abase switching frequency corresponding to a motor speed (e.g.,functional formula (equation) information which defines a relationshipbetween a motor speed and a switching frequency) may be used, and byusing the data information (previously stored in a memory of thecontroller), the controller determines the base switching frequencyF_(sw) _(—) _(base) corresponding to the current motor speed W_(rpm).

The data information may be similar with that of the first embodimentshown in FIG. 2, in which the base switching frequency F_(sw) _(—)_(base) is set higher as the motor speed W_(rpm) increases.

That is, as the motor speed (an absolute value thereof) in the datainformation increases, the base switching frequency corresponding to themotor speed is proportionally set higher, and in actual vehicle control,the base switching frequency is determined from the data information tobe a value which continuously changes proportionally to the motor speedchange.

As such, once the base switching frequency F_(sw) _(—) _(base) isdetermined according to the motor speed W_(rpm), the base switchingfrequency F_(sw) _(—) _(base) is compared with a previously setreference frequency F_(SD) in step S12′. When the base switchingfrequency F_(sw) _(—) _(base) exceeds the reference frequency F_(SD),the switching frequency F_(sw) actually used in inverter control isfinally determined to be the base switching frequency (F_(sw) _(—)_(base)=F_(sw)) in step S14′.

Like in the first exemplary embodiment, the inverter is controlled inthe single-sampling mode (F_(samp)=F_(sw)) in which the samplingfrequency F_(samp) is determined to be equal to the switching frequencyF_(sw) for use in step S14″.

The second exemplary embodiment further provides sophisticatedlysegmented control logic in which the controller determines whether thecurrent motor operation state is in a driving operation state or aregenerating operation state and the switching frequency F_(sw) and thesampling frequency F_(samp) are determined from the base switchingfrequency F_(sw) _(—) _(base) separately for the driving operation stateand the regenerating operation state. In addition, in the secondembodiment, it is determined whether to conduct step transition of theswitching frequency and one of the single-sampling mode and thedouble-sampling mode is selected, according to the current inverterinput voltage, motor speed, torque command, or inverter power.

That is, when the motor is in the driving operation state, when threeconditions all are satisfied: i.) an inverter input voltage VDC isgreater than a preset first reference voltage VDC_cal1; ii.) the motorspeed W_(rpm) (e.g., an absolute value thereof) is a value in a presetfirst speed range (a value between a first reference speed W_(rpm) _(—)_(cal1) and a second reference speed W_(rpm) _(—) _(cal2))_(;) and iii.)an absolute value of a torque command, |T_(e)*| is greater than a firstreference torque T_(e) _(—) _(cal1) or an absolute value of the inverterpower, |Power|, is greater than a first reference power Power_cal1, thenthe switching frequency F_(sw) is finally determined to be two times thebase switching frequency, 2×F_(sw) _(—) _(base) and step transition ofthe switching frequency used in inverter control (F_(sw)=F_(sw) _(—)_(base)) is conducted in steps S16, S17, S18, and S19-1. The samplingfrequency F_(samp) is set to a frequency that is equal to a frequency ofthe single-sampling mode (F_(samp)=F_(sw)), that is, the switchingfrequency for use in inverter control, in step S19-2.

On the other hand, when any one of the three conditions is notsatisfied, the switching frequency is finally determined to the baseswitching frequency without step transition of the switching frequency(F_(sw)=F_(sw) _(—) _(base)), in step S20-1, and in this case, for thesampling frequency F_(samp), control is performed in the double-samplingmode (F_(samp)=2×F_(sw)) in step S20-2.

In other words, frequency which is two times the switching frequency isdetermined as the sampling frequency and is used for inverter control.

When the motor is in the regenerating operation state, three conditionsall are satisfied: the inverter input voltage VDC is greater than apreset second reference voltage VDC_cal2; the motor speed W_(rpm) (anabsolute value thereof) is a value in a preset second speed range (avalue between a third reference speed W_(rpm) _(—) _(cal3) and a fourthreference speed W_(rpm) _(—) _(cal4)); and an absolute value of a torquecommand, |T_(e)*| is greater than a second reference torque T_(e) _(—)_(cal2) or an absolute value of the inverter power, |Power|, is greaterthan a second reference power Power_cal2, then the switching frequencyF_(sw) is finally determined to be two times the base switchingfrequency, 2×F_(sw) _(—) _(base) and step transition of the switchingfrequency used in inverter control (F_(sw)=F_(sw) _(—) _(base)) isconducted in steps S16′, S17′, S18′, and S19′-1.

The sampling frequency F_(samp) is set to a frequency that is equal to afrequency of the single-sampling mode (F_(samp)=F_(sw)), that is, theswitching frequency for use in inverter control, in step S19′-2.

When any one of the three conditions is not satisfied, the switchingfrequency is finally determined to the base switching frequency withoutstep transition of the switching frequency (F_(sw)=F_(sw) _(—) _(base)),in step S20′-1, and in this case, for the sampling frequency F_(samp),control is performed in the double-sampling mode (F_(samp)=2×F_(sw)) instep S20′-2.

In this way, the switching frequency and the sampling frequency may bedetermined from the base switching frequency separately for the drivingoperation and the regenerating operation of the motor, and in this case,for application to the motor's driving operation and regeneratingoperation, the reference voltage (i.e., the first reference voltage andthe second reference voltage, respectively), the speed range (i.e., thefirst speed range and the second speed range, respectively), thereference torques (i.e., the first reference torque and the secondreference torque, respectively), and the reference power (i.e., thefirst reference power and the second reference power, respectively) maybe set to different values, respectively.

Thus, in the second exemplary embodiment, according to current motoroperation state information regarding the driving operation and theregenerating operation, which includes the inverter input voltage, themotor speed, torque command, or inverter power, step transition of theswitching frequency and single/double sampling transition are madevariably.

Referring to FIG. 5, the three conditions are satisfied in aparticular-speed area of the motor, such that step transition of theswitching frequency F_(sw) is made, and in this case, the switchingfrequency has a control pattern which discontinuously varies.

In contrast to the first exemplary embodiment in which discontinuoussingle/double sampling transition is made as shown in FIG. 3, in thesecond exemplary embodiment, a continuous sampling pattern as shown inFIG. 6 is provided by single/double sampling transition made in theforegoing manner.

Also in the second exemplary embodiment, the switching frequency isproperly changed according to the motor operation state and propertransition between the double-sampling mode and the single-sampling modeis made for the sampling frequency, thereby achieving overallimprovements in terms of switching loss, electromagnetic performance,NVH performance, control stability, and so forth when compared to aconventional case in which one switching frequency and one samplingfrequency are used over the entire operation area.

Next, a third exemplary embodiment of the present invention will bedescribed.

FIGS. 7A and 7B are diagrams illustrating a base switching frequency anda spread frequency according to the third exemplary embodiment of thepresent invention, and FIG. 8 is a flowchart illustrating an invertercontrol method according to the third exemplary embodiment of thepresent invention.

In the third exemplary embodiment of the present invention, a baseswitching frequency F_(sw) _(—) _(base) and a spread frequency F_(sw)_(—) _(inj) both are frequency values that are obtained from a currentmotor speed W_(rpm), and in the current exemplary embodiment, theconcept of a spread frequency is introduced. That is, in the thirdexemplary embodiment of the present invention, the spread frequencyF_(sw) _(—) _(inj) corresponding to the current motor speed W_(rpm) isadditionally reflected to obtain a switching frequency. This switchingfrequency is obtained by summing the base switching frequency F_(sw)_(—) _(base) and the spread frequency F_(sw) _(—) _(inj) that are todetermined according to a motor speed.

Thus, the switching frequency obtained by summing the base switchingfrequency F_(sw) _(—) _(base) and the spread frequency F_(sw) _(—)_(inj) is also determined according to the current motor speed, and inthe following description, the switching frequency obtained by summingthe base switching frequency F_(sw) _(—) _(base) and the spreadfrequency F_(sw) _(—) _(inj) will be indicated by ‘F_(sw) _(—) _(nom)’.

An equation for calculating the switching frequency F_(sw) _(—) _(nom)may be expressed as follows:

F _(sw) _(—) _(nom) =f1(|W _(rpm)|)+f2(|W _(rpm)|)=f(|W _(rpm)|)=F _(sw)_(—) _(base) +F _(sw) _(—) _(inj)  (1)

wherein F_(sw) _(—) _(base)=f1(|W_(rpm)|), and F_(sw) _(—)_(inj)=f2(|W_(rpm)|).

The base switching frequency F_(sw) _(—) _(base) is a switchingfrequency that is continuous-variable-controlled according to the motorspeed W_(rpm) like in FIG. 2 according to the first exemplaryembodiment, such that the controller determines the base switchingfrequency F_(sw) _(—) _(base) from the current motor speed W_(rpm).

In this case, data information which defines the base switchingfrequency F_(sw) _(—) _(base) as a value corresponding to a motor speed(or equation information which defines a relationship between a motorspeed and a base switching frequency) may be used, and by using thisdata information (e.g., stored in a memory or storage device of thecontroller in advance), the controller obtains the base switchingfrequency F_(sw) _(—) _(base) corresponding to the current motor speedW_(rpm).

FIG. 7A illustrates a state in which the base switching frequency F_(sw)_(—) _(base) is continuous-variable-controlled according to the motorspeed W_(rpm), in which a conventionally fixed switching frequency value(8 kHz) is also indicated. As illustrated in FIG. 7A, the base switchingfrequency F_(sw) _(—) _(base) may be set higher as the motor speedW_(rpm) increases. That is, as the motor speed (e.g., an absolute valuethereof) in the data information increases, the base switching frequencycorresponding to the motor speed is proportionally set higher, and inactual vehicle control, the base switching frequency is determined fromthe data information to be a value which continuously changesproportionally to the motor speed change.

As such, in the third exemplary embodiment, the switching frequencyF_(sw) _(—) _(nom) for determining the sampling frequency is obtained byfurther reflecting the spread frequency F_(sw) _(—) _(inj), that is bysumming the base switching frequency F_(sw) _(—) _(base), which iscontinuous-variable-controlled according to the motor speed, and thespread frequency F_(sw) _(—) _(inj), which is also determined accordingto the motor speed.

In this way, in the third exemplary embodiment of the present invention,the switching frequency is changed according to the motor speed byreflecting the spread frequency, thus achieving NVH performanceimprovement with frequency spreading (a noise source is dispersed byreal-time switching frequency change).

In the third exemplary embodiment, the spread frequency F_(sw) _(—)_(inj) may be obtained using a predetermined equation, for example, acosine function including a variable M_(inj) like Equation 2, in whichM_(inj) is a value determined according to a motor speed.

F _(sw) _(—) _(inj) =M _(inj)COS(2πf _(inj) t)  (2),

wherein f_(inj) indicates a previously set constant and t indicates atime variable.

FIG. 7B illustrates an example in which M_(inj) is set as a valuecorresponding to the motor speed W_(rpm). In the third exemplaryembodiment of the present invention, as illustrated in FIG. 7B, M_(inj)is previously set according to the motor speed W_(rpm) and then by usingM_(inj) that is obtained from the real-time changing motor speedW_(rpm), the spread frequency F_(sw) _(—) _(inj) is obtained based onEquation 2.

FIG. 9 is a diagram illustrating a switching scheme for the samplingfrequency F_(samp) in an inverter control method according to theexemplary third embodiment of the present invention. As illustrated inFIG. 9, in the third exemplary embodiment, by using the switchingfrequency F_(sw) _(—) _(nom) obtained by summing the base switchingfrequency F_(sw) _(—) _(base) and the spread frequency F_(sw) _(—)_(inj), the sampling frequency F_(samp) is determined.

At this time, a scheme in which the sampling frequency F_(samp) ischanged according to the switching frequency F_(sw) _(—) _(nom) andtransition (e.g., single sampling

double sampling) is performed in a particular condition is the same asthe first exemplary embodiment, except that determination of thesampling frequency F_(samp) uses the switching frequency F_(sw) _(—)_(nom) obtained by summing the base switching frequency F_(sw) _(—)_(base) and the spread frequency F_(sw) _(—) _(inj).

With reference to FIG. 8, a description will be made below regarding aprocess of performing transition between single sampling and doublesampling based on a motor speed in the inverter control method accordingto the third embodiment of the present invention.

First, the controller monitors the current motor speed W_(rpm) andvariable-controls the inverter switching frequency F_(sw) _(—) _(nom)according to the current motor speed W_(rpm). The switching frequencyF_(sw) _(—) _(nom) is obtained by summing the base switching frequencyF_(sw) _(—) _(base) and the spread frequency F_(sw) _(—) _(inj) asstated above, and both the base switching frequency F_(sw) _(—) _(base)and the spread frequency F_(sw) _(—) _(inj) are calculated as valueschanging with the motor speed W_(rpm), such that when the motor speedW_(rpm) changes, the switching frequency F_(sw) _(—) _(nom) also changescorrespondingly.

Referring to FIG. 8, in step S11 where the switching frequency F_(sw)_(—) _(nom) is calculated as a value corresponding to the motor speedW_(rpm), the switching frequency F_(sw) _(—) _(nom) is determined as afunction of a motor speed absolute value |W_(rpm)|. As such, once theswitching frequency change value corresponding to the motor speed isdetermined, on/off driving of a switching element (an IGBT of an IGBTpower module) in an inverter is controlled, which inverts direct current(DC) power into a three-phase alternating current (AC) for motor drivingby generating a triangle-wave using the changed switching frequency andgenerating a pulse width modulation (PWM) signal.

The sampling frequency F_(samp) is set variably according to theswitching frequency F_(sw) _(—) _(nom) determined by the motor speedW_(rpm) and the switching frequency F_(sw) _(—) _(nom) is compared withthe preset reference frequency F_(SD) in step S12. If the switchingfrequency is less than the reference frequency, the inverter iscontrolled in the double sampling mode F_(samp)=2×F_(sw) _(—) _(nom)which uses a frequency that is two times the switching frequency as thesampling frequency in step S13.

On the other hand, in an area where the switching frequency F_(sw) _(—)_(nom) exceeds the reference frequency F_(SD), the sampling frequencyF_(samp) is determined to be the same frequency as the switchingfrequency in step S14, and the inverter is controlled in thesingle-sampling mode (F_(samp)=F_(sw) _(—) _(nom)) which uses the samefrequency as the switching frequency as the sampling frequency.

Herein, the reference frequency serves as a criterion for transitionbetween the single-sampling mode (F_(samp)=F_(sw)) and thedouble-sampling mode (F_(samp)=2×F_(sw)), and the reference frequency ispreviously determined after a pre-test with respect to a motor systemhaving the same specifications.

Referring to FIG. 9, in an area where the switching frequency F_(sw)_(—) _(nom) is lower than the reference frequency F_(SD), and in an areawhere the switching frequency F_(sw) _(—) _(nom) exceeds the referencefrequency F_(SD), the inverter is controlled in the double-sampling modeand the single-sampling mode, respectively. As such, in the thirdexemplary embodiment of the present invention, the switching frequencycontinuously varies with the motor speed and mode transition betweendouble sampling and single sampling is made based on a particular speedof the motor (a particular switching frequency).

Next, a fourth exemplary embodiment of the present invention will bedescribed.

FIG. 10 is a flowchart illustrating an inverter control method accordingto the fourth exemplary embodiment of the present invention, and FIG. 11is a diagram schematically illustrating a state where the switchingfrequency is variable-controlled (step-transition-controlled) in theinverter control method and that single sampling/double samplingtransition is made as step transition of the switching frequency is madein the inverter control method according to the fourth exemplaryembodiment of the present invention.

The fourth exemplary embodiment of the present invention also uses aswitching frequency corresponding to a motor speed described in thethird exemplary embodiment, that is, the switching frequency F_(sw) _(—)_(nom) obtained by summing the base switching frequency F_(sw) _(—)_(base) and the spread frequency F_(sw) _(—) _(inj) which are determinedby respective calculation methods from the motor speed W_(rpm).

The fourth exemplary embodiment is not quite different from the secondexemplary embodiment in terms of an overall control method, except thatthe switching frequency F_(sw) _(—) _(nom) obtained by summing the baseswitching frequency F_(sw) _(—) _(base) and the spread frequency F_(sw)_(—) _(inj) is used instead of the base switching frequency of thesecond exemplary embodiment.

First, as illustrated in FIG. 10, the controller determines theswitching frequency F_(sw) _(—) _(nom) obtained by summing the baseswitching frequency F_(sw) _(—) _(base) and the spread frequency F_(sw)_(—) _(inj) from the current motor speed W_(rpm), while monitoring thecurrent motor speed W_(rpm), in step S11′.

Thereafter, once the switching frequency F_(sw) _(—) _(nom)corresponding to the current motor speed W_(rpm) is determined, thecontroller compares the switching frequency F_(sw) _(—) _(norm) with thepredetermined reference frequency F_(SD) in step S12′. If the baseswitching frequency exceeds the reference frequency F_(SD), thecontroller finally determines the switching frequency F_(sw) actuallyused in inverter control as the switching frequency F_(sw) _(—) _(nom)(F_(sw)=F_(sw) _(—) _(nom)) in step S14′.

In addition, the controller controls the inverter in the single samplingmode (F_(samp)=F_(sw)) which determines the sampling frequency F_(samp)to be a frequency that is equal to the switching frequency F_(sw) anduses the determined frequency in step S14″.

Like the second exemplary embodiment, the fourth exemplary embodimentalso provides sophisticatedly segmented control logic in which thecontroller determines whether the current motor operation state is in adriving operation state or a regenerating operation state and the finalswitching frequency F_(sw) and sampling frequency F_(samp) aredetermined from the base switching frequency F_(sw) _(—) _(nom) that isa sum of the base switching frequency F_(sw) _(—) _(base) and the spreadfrequency F_(sw) _(—) _(inj), separately for the driving operation stateand the regenerating operation state. In addition, it is determinedwhether to conduct step transition of the switching frequency and one ofthe single-sampling mode and the double-sampling mode is selected,according to the current inverter input voltage, motor speed, torquecommand, or inverter power.

That is, when the motor is in the driving operation state, when threeconditions all are satisfied: i.) an inverter input voltage VDC isgreater than a preset first reference voltage VDC_cal1; ii.) the motorspeed W_(rpm) (e.g., an absolute value thereof) is a value in a presetfirst speed range (a value between a first reference speed W_(rpm) _(—)_(cal1) and a second reference speed W_(rpm) _(—) _(cal2)); and iii.) anabsolute value of a torque command, |T_(e)*| is greater than a firstreference torque T_(e) _(—) _(cal1) or an absolute value of the inverterpower, |Power|, is greater than a first reference power Power_cal1, thenthe switching frequency F_(sw) is finally determined to be two times thebase switching frequency, 2×F_(sw) _(—) _(base), and the switchingfrequency F_(sw) is finally determined as a value that is two times theswitching frequency F_(sw) _(—) _(nom), 2×F_(sw) _(—) _(nom), and steptransition of the switching frequency used in inverter control(F_(sw)=2×F_(sw) _(—) _(nom)) is conducted in steps S16, S17, S18, andS19-1.

The sampling frequency F_(samp) is set to a frequency that is equal to afrequency of the single-sampling mode (F_(samp)=F_(sw)), that is, theswitching frequency F_(sw) for use in inverter control, in step S19-2.

On the other hand, when any one of the three conditions is notsatisfied, the switching frequency F_(sw) is finally determined to bethe switching frequency F_(sw) _(—) _(nom) that is a sum of the baseswitching frequency F_(sw) _(—) _(base) and the spread frequency F_(sw)_(—) _(inj) without step transition of the switching frequency(F_(sw)=F_(sw) _(—) _(nom)), in step S20-1, and in this case, for thesampling frequency F_(samp), control is performed in the double-samplingmode (F_(samp)=2×F_(sw)) in step S20-2.

When the motor is in the regenerating operation state, three conditionsall are satisfied: the inverter input voltage VDC is greater than apreset second reference voltage VDC_cal2; the motor speed W_(rpm) (anabsolute value thereof) is a value in a preset second speed range (avalue between a third reference speed W_(rpm) _(—) _(cal3) and a fourthreference speed W_(rpm) _(—) _(cal4)); and an absolute value of a torquecommand, |T_(e)*| is greater than a second reference torque T_(e) _(—)_(cal2) or an absolute value of the inverter power, |Power|, is greaterthan a second reference power Power_cal2, then the switching frequencyF_(sw) is finally determined to be two times the base switchingfrequency, 2×F_(sw) _(—) _(base) and step transition of the switchingfrequency used in inverter control (F_(sw)=2×F_(sw) _(—) _(nom)) isconducted in steps S16′, S17′, S18′, and S19′-1.

The sampling frequency F_(samp) is set to a frequency that is equal to afrequency of the single-sampling mode (F_(samp)=F_(sw)), that is, theswitching frequency for use in inverter control, in step S19′-2.

When any one of the three conditions is not satisfied, the switchingfrequency F_(sw) is finally determined to the switching frequency F_(sw)_(—) _(nom) obtained by summing F_(sw) _(—) _(base) and the spreadfrequency F_(sw) _(—) _(inj) without step transition of the switchingfrequency (F_(sw)=F_(sw) _(—) _(nom)), in step S20′-1, and in this case,for the sampling frequency F_(samp), control is performed in thedouble-sampling mode (F_(samp)=2×F_(sw)) in step S20′-2.

In this way, the switching frequency and the sampling frequency may bedetermined from the base switching frequency separately for the drivingoperation and the regenerating operation of the motor, and in this case,for application to the motor's driving operation and regeneratingoperation, the reference voltage (i.e., the first reference voltage andthe second reference voltage, respectively), the speed range (i.e., thefirst speed range and the second speed range, respectively), thereference torques (i.e., the first reference torque and the secondreference torque, respectively), and the reference power (i.e., thefirst reference power and the second reference power, respectively) maybe set to different values, respectively.

The first reference voltage and the second reference voltage, the firstspeed range and the second speed range, the first reference torque andthe second reference torque, and the first reference output and thesecond reference output of the fourth exemplary embodiment may be equalto or different from those of the second exemplary embodiment.

As such, in the fourth exemplary embodiment, according to the currentmotor operation state information including the current motor operationstate information classified into the driving state and the regeneratingstate, the inverter input voltage and motor rotation speed, and torquecommand or inverter power, switching frequency step transition andsingle/double sampling transition are variably performed.

Referring to FIG. 11A, the foregoing three conditions all are satisfiedin the motor's particular speed area, such that step transition of theswitching frequency F_(sw) is performed and the switching frequency hasa control pattern that discontinuously varies.

Unlike in the third exemplary embodiment where discontinuoussingle/double sampling transition is made as illustrated in FIG. 9, inthe fourth exemplary embodiment, a continuous sampling pattern as shownin FIG. 11B is provided by single/double sampling transition made in theforegoing manner.

FIG. 12 and FIGS. 13A and 13B are diagrams illustrating a fifthexemplary embodiment of the present invention. FIG. 12 is a flowchartillustrating an inverter control method according to the fifth exemplaryembodiment of the present invention, and FIGS. 13A and 13B are diagramsschematically illustrating a state in which a switching frequency isvariable-controlled (step-transition-controlled) and singlesampling/double sampling transition is performed by step transition ofthe switching frequency in an inverter control method according to thefifth exemplary embodiment of the present invention.

Like the fourth exemplary embodiment, the fifth exemplary embodiment ofthe present invention uses the switching frequency F_(sw) _(—) _(nom)that is a sum of the base switching frequency F_(sw) _(—) _(base) andthe spread frequency F_(sw) _(—) _(inj). The fifth exemplary embodimentis not different from the fourth exemplary embodiment, except for torqueand power conditions for determining to perform step transition in stepsS19-1 and S19′-1 and not to perform step transition in steps S20-1 andS20′-1.

In the fifth exemplary embodiment, the reference voltage, the speedrange, the reference torque, and the reference power used in steps S16,S16′, S17, S17′, S18, and S18′ may be equal to or different from thoseof the fourth exemplary embodiment, and in FIG. 12, they are indicatedby different symbols (VDC_cal3, VDC_cal4, W_(rpm) _(—) _(cal5), W_(rpm)_(—) _(cal6), W_(rpm) _(—) _(cal7), W_(rpm) _(—) _(cal8), T_(e) _(—)_(cal3), T_(e) _(—) _(cal3), Power_cal3, or Power_cal4) on theassumption that they are different from those of the fourth exemplaryembodiment.

In the fifth exemplary embodiment, when a condition that an absolutevalue of a torque command, |T_(e)*|, is less than a third referencetorque T_(e) _(—) _(cal3) or a condition that an absolute value of aninverter power, |Power|, is less than a third reference powerPower_cal3, is satisfied, the switching frequency F_(sw) is finallydetermined to be a frequency (2×F_(sw) _(—) _(nom)) that is two timesthe switching frequency F_(sw) _(—) _(nom) and step transition of aswitching frequency used in inverter control (F_(sw)=2×F_(sw) _(—)_(nom)) is performed in step S19-1.

The sampling frequency F_(samp) is set to a frequency that is equal to afrequency of the single-sampling mode (F_(samp)=F_(sw)), that is, theswitching frequency F_(sw) for use in inverter control, in step S19-2.

On the other hand, if the torque condition or the inverter powercondition is not satisfied in step S18, the switching frequency F_(sw)is finally determined to be the switching frequency F_(sw) _(—) _(nom)that is a sum of the base switching frequency F_(sw) _(—) _(base) andthe spread frequency F_(sw) _(—) _(inj) without step transition of theswitching frequency in step S20-1, and the sampling frequency F_(samp)is controlled in the double-sampling mode (F_(samp)=2×F_(sw)) in stepS20-2.

If a condition that the absolute value of the torque command, |T_(e)*|,is less than a fourth reference torque T_(e) _(—) _(cal4) or a conditionthat an absolute value of the inverter power, |Power|, is less than afourth reference power Power_cal4, is satisfied in step S18′, theswitching frequency F_(sw) is finally determined to be a frequency thatis two times the switching frequency F_(sw) _(—) _(nom) and steptransition of the switching frequency used in inverter to control isperformed (F_(sw)=2×F_(sw) _(—) _(nom)) in step S19′-1.

The sampling frequency F_(samp) is set to a frequency that is equal tothe frequency of the single sampling mode (F_(samp)=F_(sw)), that is,the switching frequency, for use in inverter control in step S19′-2.

On the other hand, if the torque condition or the inverter powercondition is not satisfied in step S18′, the switching frequency F_(sw)is finally determined to be the switching frequency F_(sw) _(—) _(nom)that is a sum of the base switching frequency F_(sw) _(—) _(base) andthe spread frequency F_(sw) _(—) _(inj) without step transition of theswitching frequency (F_(sw)=F_(sw) _(—) _(nom)) in step S20′-1, and inthis case, the sampling frequency F_(samp) is controlled in the doublesampling mode (F_(samp)=2×F_(sw)) in step S20′-2.

Referring to FIG. 13A, step transition of the switching frequency F_(sw)is made in a particular speed area of the motor according to the processillustrated in FIG. 12, and the switching frequency has a controlpattern that discontinuously varies.

Unlike in the third exemplary embodiment where discontinuoussingle/double sampling transition is made as illustrated in FIG. 9, inthe fifth exemplary embodiment, a continuous sampling pattern isprovided by single/double sampling transition as illustrated in FIG.13B.

In this way, as described in the fourth exemplary embodiment and thefifth exemplary embodiment, in the inverter control process of thepresent invention, if one of the condition of step S18 in FIG. 10 andthe condition of step S18 in FIG. 12 is satisfied, steps S19-1 and S19-2are performed, and if none of the condition of step S18 in FIG. 10 andthe condition of step S18 in FIG. 12 is satisfied, steps S20-1 and S20-2may be performed.

In this case, as described above, if one of the condition of step S18′in FIG. 10 and the condition of step S18′ in FIG. 12 is satisfied, stepsS19′-1 and S19′-2 are performed, and if none of the condition of stepS18′ in FIG. 10 and the condition of step S18′ in FIG. 12 is satisfied,steps S20′-1 and S20′-2 may be performed.

FIG. 14 is a diagram illustrating a step transition area of a switchingfrequency according to an exemplary embodiment of the present invention.

Therefore, with the inverter control method according to the presentinvention, the switching frequency is properly changed according to themotor operation state and proper transition between the double-samplingmode and the single-sampling mode is made for the sampling frequency,thereby achieving overall improvements in terms of switching loss,electromagnetic performance, NVH performance, control stability, and soforth when compared to a conventional case in which one switchingfrequency and one sampling frequency are used over the entire operationarea.

While the embodiments of the present invention have been described indetail, the scope of the present invention is not limited to theforegoing embodiments, and various modifications and improvements madeby those of ordinary skill in the art by using the base concept of thepresent invention defined in the appended claims are also included inthe scope of the present invention.

What is claimed is:
 1. An inverter control method for an eco-friendlyvehicle which generates a pulse width modulation (PWM) signal accordingto a switching frequency and a sampling frequency and controls ON/OFFdriving of a switching element, wherein a controller changes and setsthe switching frequency according to a current motor speed, changes andsets a sampling frequency according to the switching frequency, andcontrols on/off driving of a switching element according to theswitching frequency corresponding to the current motor speed and thesampling frequency.
 2. The inverter control method of claim 1, whereinthe switching frequency is changed and set in proportion to a change ofthe motor speed.
 3. The inverter control method of claim 1, wherein theswitching frequency is set higher as the motor speed increases.
 4. Theinverter control method of claim 1, wherein the switching frequency iscompared with a predetermined reference frequency, such that when theswitching frequency is lower than the predetermined reference frequency,double-sampling control is performed in which a frequency which is twotimes the switching frequency that is set according to the current motorspeed is used as the sampling frequency, and when the switchingfrequency exceeds the predetermined reference frequency, single-samplingcontrol is performed in which a frequency which is equal to theswitching frequency is used as the sampling frequency.
 5. The invertercontrol method of claim 1, wherein the switching frequency is obtainedby summing a base switching frequency and a spread frequency that aredetermined according to the current motor speed.
 6. The inverter controlmethod of claim 5, wherein the base switching frequency is set tocontinuously vary according to a change of the motor speed.
 7. Theinverter control method of claim 5, wherein the base switching frequencyis set higher as the motor speed increases.
 8. The inverter controlmethod of claim 5, wherein the spread frequency is obtained by thefollowing equation comprising a variable M_(inj) determined according tothe current motor speed:F _(sw) _(—) _(inj) =M _(inj)COS(2πf _(inj) t) wherein F_(sw) _(—)_(inj) indicates a spread frequency, M_(inj) indicates a variable thatis preset to a value corresponding to a motor speed, f_(inj) indicates apreset constant, and t indicates a time variable.
 9. An inverter controlmethod for an eco-friendly vehicle which generates a pulse widthmodulation (PWM) signal according to a switching frequency and asampling frequency and controls ON/OFF driving of a switching element,wherein after a controller determines a base switching frequencyaccording to a current motor speed, the controller changes and sets theswitching frequency and a sampling frequency to values corresponding toa current motor operation state from the base switching frequency, andcontrols on/off driving of a switching element according to theswitching frequency and the sampling frequency.
 10. The inverter controlmethod of claim 9, wherein the base switching frequency is changed andset in proportion to a change of the motor speed.
 11. The invertercontrol method of claim 9, wherein the base switching frequency is sethigher as the motor speed increases.
 12. The inverter control method ofclaim 9, wherein the base switching frequency is compared with apredetermined reference frequency, such that when the base switchingfrequency is lower than the predetermined reference frequency, theswitching frequency and the sampling frequency are changed and set tovalues corresponding to current motor operation state information. 13.The inverter control method of claim 9, wherein if the base switchingfrequency exceeds the predetermined reference frequency, a frequencyequal to the base switching frequency is determined and used as theswitching frequency and the sampling frequency.
 14. The inverter controlmethod of claim 12, wherein the motor operation state informationcomprises an inverter input voltage, a motor speed, a torque command, oran inverter power.
 15. The inverter control method of claim 14, whereinwhen a condition in which the inverter input voltage is greater than areference voltage, a condition in which the motor speed is in apredetermined speed range, and a condition in which an absolute value ofthe torque command is greater than a reference torque or an absolutevalue of the inverter power is greater than a reference power are allsatisfied, then the switching frequency is set to a frequency that istwo times the base switching frequency and the sampling frequency is setto a frequency that is equal to the switching frequency.
 16. Theinverter control method of claim 14, wherein if any one of a conditionin which the inverter input voltage is greater than a reference voltage,a condition in which the motor speed is in a predetermined speed range,and a condition in which an absolute value of the torque command isgreater than a reference torque or an absolute value of the inverterpower is greater than a reference power is not satisfied, then theswitching frequency is set to a frequency that is equal to the baseswitching frequency and the sampling frequency is set to a frequencythat is two times the switching frequency.
 17. The inverter controlmethod of claim 15, wherein the motor operation state informationfurther comprises driving operation/regenerating operation states of themotor, and the controller is configured to determine whether the currentmotor operation state is the driving operation state or the regeneratingoperation state and to determine whether the conditions are satisfied inthe determined operation state, and for application to the motor'sdriving operation and regenerating operation, the reference voltage, thespeed range, the reference torque, and the reference power are set todifferent values, respectively, by the controller.
 18. The invertercontrol method of claim 9, wherein the controller further determines aspread frequency according to the current motor speed, and changes andsets the switching frequency and the sampling frequency to a valuecorresponding to the current motor operation state by using theswitching frequency obtained by summing the base switching frequency andthe spread frequency.
 19. The inverter control method of claim 18,wherein the spread frequency is obtained by the following equationcomprising a variable M_(inj) determined according to the current motorspeed:F _(sw) _(—) _(inj) =M _(inj)COS(2πf _(inj) t) wherein F_(sw) _(—)_(inj) indicates a spread frequency, M_(inj) indicates a variable thatis preset to a value corresponding to a motor speed, f_(inj) indicates apreset constant, and t indicates a time variable.
 20. The invertercontrol method of claim 18, wherein the switching frequency obtained bysumming the base switching frequency and the spread frequency iscompared with a predetermined reference frequency, such that when theswitching frequency is lower than the predetermined reference frequency,the switching frequency and the sampling frequency are changed and setto a value corresponding to the current motor operation state.
 21. Theinverter control method of claim 18, wherein when the switchingfrequency obtained by summing the base switching frequency and thespread frequency exceeds the predetermined reference frequency, afrequency which is equal to the switching frequency obtained by summingthe base switching frequency and the spread frequency is finallydetermined and used as the switching frequency and the samplingfrequency.
 22. The inverter control method of claim 20, wherein themotor operation state information comprises an inverter input voltage, amotor speed, a torque command, or an inverter power.
 23. The invertercontrol method of claim 22, wherein when a condition in which theinverter input voltage is greater than a reference voltage, a conditionin which the motor speed is in a predetermined speed range, and acondition in which an absolute value of the torque command is greaterthan a reference torque or an absolute value of the inverter power isgreater than a reference power are all satisfied, then the switchingfrequency is finally determined as and set to a frequency that is twotimes the switching frequency obtained by summing the base switchingfrequency and the spread frequency, and the sampling frequency is set toa frequency that is equal to the finally determined switching frequency.24. The inverter control method of claim 22, wherein if any one of acondition in which the inverter input voltage is greater than areference voltage, a condition in which the motor speed is in apredetermined speed range, and a condition in which an absolute value ofthe torque command is greater than a reference torque or an absolutevalue of the inverter power is greater than a reference power is notsatisfied, then the switching frequency is finally determined and set toa frequency that is equal to the switching frequency obtained by summingthe base switching frequency and the spread frequency, and the samplingfrequency is set to a frequency that is two times the finally determinedswitching frequency.
 25. The inverter control method of claim 22,wherein when a condition in which the inverter input voltage is greaterthan a reference voltage, a condition in which the motor speed is in apredetermined speed range, and a condition in which an absolute value ofthe torque command is less than a reference torque or an absolute valueof the inverter power is less than a reference power are all satisfied,then the switching frequency is finally determined as and set to afrequency that is two times the switching frequency obtained by summingthe base switching frequency and the spread frequency, and the samplingfrequency is set to a frequency that is equal to the finally determinedswitching frequency.
 26. The inverter control method of claim 22,wherein if any one of a condition in which the inverter input voltage isgreater than a reference voltage, a condition in which the motor speedis in a predetermined speed range, and a condition in which an absolutevalue of the torque command is less than a reference torque or anabsolute value of the inverter power is less than a reference power isnot satisfied, then the switching frequency is finally determined andset to a frequency that is equal to the switching frequency obtained bysumming the base switching frequency and the spread frequency, and thesampling frequency is set to a frequency that is two times the finallydetermined switching frequency.
 27. The inverter control method of claim26, wherein the motor operation state information further comprisesdriving operation/regenerating operation states of the motor, and thecontroller is configured to determine whether the current motoroperation state is the driving operation state or the regeneratingoperation state and to determine whether the conditions are satisfied inthe determined operation state, and for application to the motor'sdriving operation and regenerating operation, the reference voltage, thespeed range, the reference torque, and the reference power are set todifferent values, respectively, by the controller.